Group-III nitride (often referred to as III-nitride or III-N) compounds, such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting and laser diodes, and UV photo-detectors. The large bandgap and high electron saturation velocity of the III-nitride compounds also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material whose lattice constant and thermal expansion coefficient closely match that of GaN.
SiC is a semiconductor material providing excellent thermal conductivity, but is very expensive and available only in small wafer sizes. Direct growth of GaN on SiC is generally difficult due to poor wetting between these materials. Although buffer layers, such as AlN or AlGaN, can be used to address this wetting problem, such buffer layers cause an increase in the resistance between the overlying device and the underlying substrate. In addition, it is very difficult to prepare a SiC layer having a smooth surface, while a rough interface with GaN can cause an increase in defect density of the GaN layer.
The most highly refined semiconductor substrates are silicon substrates, which have also been considered for the growth of GaN films. Silicon substrates for GaN growth is attractive given its low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon wafers promises easy integration of GaN-based optoelectronic devices with Si-based electronic devices.
Among the feasible substrates, silicon (100) substrate is well known in the art of CMOS circuits. However, silicon (100) has a great lattice mismatch with GaN. As a result, the GaN films grown on silicon (100) substrates have polycrystalline structures, instead of the desirable crystalline structures. On the other hand, silicon (111) substrates are more suitable for forming GaN films due to their trigonal symmetry. However, silicon (111) substrates are rarely used for conventional CMOS applications, and hence have relatively higher cost and less availability. The above discussed options require compromise to be made between cost, performance, and/or process complexity. New methods for forming GaN films are thus needed to solve the above discussed problems.